During the early 1980's improvements in computer technology greatly facilitated the description, manipulation, and representation of complex three-dimensional information in digital form. This progress was particularly dramatic in the area of computer aided design (CAD). Methods of physical fabrication, however, were almost exclusively limited to old subtractive methods which were adapted for computer numerical control (CNC). Consequently, a number of people noted the great disparity between the ability to describe and represent complex three-dimensional information in digital, nonphysical, form and the ability to turn those descriptions into correspondingly detailed physical forms. This disparity between nonphysical and physical capabilities led an increased number of researchers to focus on a variety of new techniques which promised to provide fast and economical three-dimensional "hard copy," with little or no constraint on complexity.
Stereolithography, the first commercially successful method of "freeform fabrication," was first publicly demonstrated in late 1987. Since then a wide variety of new methods have appeared. Most of these new methods are performed in a layer-by-layer fashion and are characterized by their additive nature, either solidifying or depositing small portions of material onto an object being produced. One of these new additive methods is commonly called laminated object manufacturing (LOM). Unlike the others, however, LOM is distinguished by the fact that it employs both additive methods (lamination of a layer) and subtractive methods (cutting of a cross-sectional shape) in order to manufacture objects.
Part of the history of freeform fabrication and LOM is outlined in U.S. Pat. No. 5,354,414 to Feygin. This patent also suggests various methods of forming objects from layers of powder and from layers of sheet material. Of particular relevance to the current application are methods of "cut-on-the-stack" lamination which were added as new material on Oct. 4, 1989. These methods eliminate most registration problems by laminating a layer of material before cutting that layer of material to shape. In some embodiments described, it was also suggested that the surrounding material be left in place, in order to help support subsequent laminations. In each sheet-larnination embodiment the thickness of the sheet material is determined and formed prior to lamination.
The lamination work which I began in early 1986 was not described in the background to U.S. Pat. No. 5,354,414. This work focused exclusively on the lamination of sheet material and led to U.S. Pat. No. 5,015,312. It also led to the paper "Three-Dimensional Printing," dated Apr. 27, 1988 and subsequent editions, which were widely and nonconfidentially distributed to people who were involved in this field. The lamination papers which I published all describe the "cut-on-the-stack" method and the advantages of cutting parting lines and leaving the surrounding material in place to form a supporting shell. In each embodiment, the thickness of the sheet material is determined and formed prior to lamination.
Since the commercial introduction of Stereolithography in 1987, the number of freeform fabrication research efforts has increased dramatically. Without attempting to cover all of the many variations, two methods should be noted here. They are "Shapemaker II," developed and marketed by Schroff Development Corporation of Mission, Kans., and "Computer-Aided Manufacturing of Laminated Engineering Materials" developed by CAM-LEM, Inc., of Cleveland, Ohio. Neither of these methods uses the cut-on-the-stack method. In each case, a cutting station is configured to prepare a single layer of material for lamination at a separate lamination station. In some embodiments, methods of five-axis cutting are used in order to vary the edge profile of individual layers for the purpose of reducing the "stair-stepping" error which is characteristic of methods which fabricate complex shapes by assembling or otherwise layering cross-sectional shapes which have square edge profiles.
The CAM-LEM, Inc. methods referenced above also include the construction of shapes from more than one material. At a special cutting station, these methods create component cross-sections from a selection of sheet materials, differing in both thickness and in composition. These cross-sections are then assembled into the product shape. In some cases, portions of individual layers are cut from sheets of specially formulated "fugitive materials." These materials are designed to provide structural support during critical phases of the fabrication process but to be reduced to a removable powder ash by heating the laminated object in an oven.
Additional methods of lamination from preformed sheet materials are included in currently pending U.S. patent application Ser. No. 08/587,103. These are hereby fully incorporated by reference herein as though set forth in full.
There are a number of drawbacks to each of the lamination methods and apparatus which have been disclosed to date. First, the drawbacks which apply to all of the methods, include:
A. Additive fabrication methods create objects by adding material in discrete increments. This creates the dilemma of either accepting undesirable discontinuities, or "stairsteps," in the surface or of reducing the increment size to a point which makes fabrication time and cost excessive for many purposes. Since laminated object manufacturing is a hybrid which also uses subtractive methods, it has the potential to entirely eliminate stairsteps and other unwanted discontinuities in shape. However, the subtractive mode is not yet fully exploited. The vertical-edge cutting of the current LOM hybrids cannot yet produce shapes which are equal in accuracy and smoothness to the shapes produced by conventional, purely subtractive, milling machines.
B. Since both the geometry and the use of laminated objects vary widely, it is advantageous to be able to vary the sheet thickness, both within one laminated object and between successive laminated objects. Since laminated objects are currently fabricated from sheetstock which is created by an entirely separate process, however, it is impractical to provide preformed sheet materials in every thickness which may be determined to be optimal for a particular layer. The conventional LOM approach to using preformed sheet materials is to supply sheet material which is thin enough for the most complicated portions of a particular project and to accept the inefficiencies of using a thinner-than-optimal material in less demanding portions of the project. When relatively simple objects are fabricated from thinner-than-optimal layers, the time penalty of unnecessary lamination is cumulative over many layers. In "cut-on-the-stack" apparatus material thickness changes are only made when absolutely required, with manual intervention. In the more complex "cut-off-the-stack" apparatus, changes in thickness may be somewhat easier, but are severely limited by the supply stock which is available.
C. One way to achieve optimum layer thickness is to use a variation on "layer-planing" operations which are fundamental to related methods of freeform fabrication, such as those employed by Sanders Prototype, Inc. of Wilton, N.H., and Cubital, Ltd. of Raanana, Israel. The LOM variation on this method would laminate a thicker sheet than required and then create a new "working plane" on a layer of optimum thickness by removing the excess portion of material. This alternative increases the percentage of raw material which must be discarded as waste.
D. When laminated layers are thin, they have little structural strength or resistance to deflection perpendicular to the slicing plane. This weakness makes support of cantilevers and bonding of layers particularly critical. This increases the need for excess material to be left in-place for structural support and increases the need for selective bonding, to facilitate detachment of the excess material. The need for support also creates special problems at internal voids.
E. The lamination of certain materials offers special functional potential beyond visualization and prototyping. Unlike the papers and plastics most commonly used in current lamination machines, the cutting and bonding characteristics of materials such as metals and green-state ceramic and metal powders may make it especially helpful to be able to adjust the relative usage of additive versus subtractive modes of fabrication. Especially when precision and smoothness are important, it may be advantageous to minimize the "additive mode" by reducing the number of laminations to a minimum, while simultaneously maximizing the "subtractive mode" by increasing the amount and sophistication of cutting done at each level.
F. With conventional LOM machinery, all automated cutting must be done layer-by-layer. Due to the difficulty of positioning and holding the product on a suitable automated subtractive cutting machine, any final shaping operations must be accomplished by hand. Such manual operations are expensive, time consuming and vulnerable to error.
G. All fabrication processes are subject to unanticipated errors. Prior-art lamination machines lack the capability to remove and replace an unacceptable layer or layers; they generally require that errors be corrected by disposal of an entire product and repetition of each and every layer.
H. When laminated objects are built from preformed sheet materials, there are special difficulties involved in uniformly pressing and bonding sheets which are nonplanar.
Second, there are drawbacks which are specific to methods which cut cross-sectional shapes at a special cutting station and then assemble composite shapes at a lamination station, including:
I. When cutting and lamination are accomplished at separate stations, as in the CAM-LEM methods, there is a separation of crucial operations. These methods require the separate "gripping" of each newly cut sheet and then the transfer, placement and lamination of that individual sheet. The many grip-and-place operations, at least one for each layer, increases the opportunity for error. For certain purposes, however, the CAM-LEM methods offer important new options, including the fabrication of individual object layers from more than one material.
J. When cutting and lamination are accomplished at separate stations, there are special problems associated with cutting severely sloped edges. These are largely due to the lack of structural strength and support of the extreme edges, where material thickness approaches zero. In the case of laser-based machines, there is sometimes the additional problem of the condensation of vaporized material on a surface which must still be bonded. Additional difficulties may be encountered in ensuring uniform support and lamination pressure where severely sloped edges must be bonded.
K. Various algorithms have been developed in order to better approximate complex surface shapes. To date, however, it appears that no such publicly disclosed method can adequately handle changes in surface slope which do not occur along slicing planes. While this may not be significant for minor slope changes, it is a potentially serious limitation at severe slope changes, such as from a positive slope to a negative slope. If such lines of change are horizontal, the slicing plane may be adjusted to correspond, but if such lines of change are sloped, as in the thread of a screw, more sophisticated cutting capability will be required.
Third, there are drawbacks which are specific to methods which exclusively "cut-on-the-stack," including:
L. When all cutting is done on-the-stack, it is most natural to cut only one layer at one time. Although it is theoretically possible to have more than one cutting tool working on that layer, there are severe practical limitations to the obvious alternatives. Consequently, cut-on-the-stack lamination is an inherently "serial" process, like other prior-art additive fabrication methods in which one layer is fabricated at a time. Unobvious methods of parallel fabrication are one subject of my pending U.S. patent application Ser. No. 08/587,103.
M. Cutting on-the-stack has limited lamination apparatus to one accessible working plane for each lamination. This is because the opposite side of each lamination is bonded to the previous layer before any operations can be performed on it. This limitation will become more significant as lamination machinery is adapted to profile cross-sections with edge cuts which are not vertical.
For all of the above reasons, there is a clear need to improve the LOM process, particularly in the areas of quality of output, efficiency, versatility, speed and reliability of production. The present invention clearly fulfills all of these needs.